Handover method and apparatus

ABSTRACT

A method and a server are described for supporting handover by a network node in a mobile communication system. A message related to a handover, including a first parameter indicating a cause for the handover, is received. A handover command message is transmitted to a first base station for a terminal to perform a handover to a second base station, when the network node determines that handover is needed based on the cause. A status report message including a second parameter indicating whether or not the cause for handover is solved is received. A reverse handover request message is transmitted to the second base station for the terminal to perform reverse handover to a target base station, when the network node determines that a reverse handover is needed. The network node determines whether the handover or the reverse handover is needed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of copending application Ser. No.13/148,056, filed on Aug. 5, 2011, which is the National Phase ofPCT/KR2010/000677 filed on Feb. 4, 2010, which claims priority under 35U.S.C. 119(e) to U.S. Provisional Application No. 61/150,785 filed onFeb. 8, 2009 and under 35 U.S.C. 119(a) to Patent Application No.10-2009-0118071 filed in Republic of Korea, on Dec. 1, 2009. The entirecontents of all of the above applications are hereby incorporated byreference into the present application.

TECHNICAL FIELD

The present invention relates to a mobile communication system.

BACKGROUND ART

In order to cope with various forums and new technologies related to the4th generation mobile communications, the 3rd Generation PartnershipProject (3GPP) who enacts the technical standards of 3G mobilecommunication systems has proceeded with studies on the Long TermEvolution/System Architecture Evolution (LTE/SAE) technologies since theend of 2004 as a part of the effort to optimize and enhance theperformance of 3GPP technologies.

The SAE led by 3GPP SA WG2 is a study on network technologies for thepurpose of determining a network structure together with the LTE work of3GPP TSG RAN and supporting mobility between heterogeneous networks. Inrecent years, the SAE has been considered one of the latest importantstandard issues in 3GPP. It is a work to develop a system supportingvarious radio access technologies on the basis of 3GPP IP systems, andhas been progressed to aim at an optimized packet based system thatminimizes a transmission delay with enhanced transmission capability.

A high-level reference model defined by 3GPP SA WG2 includes anon-roaming case and roaming cases having various scenarios, and thedetailed description thereof is given in 3GPP standard documents TS23.401 and TS 23.402. In FIG. 1, there is illustrated a structuraldiagram of a network in which the model is schematically reconfigured.

FIG. 1 is a structural diagram illustrating an evolved mobilecommunication network.

One of the distinctive characteristics of the network structure of FIG.1 is that it is based on a 2-tier model having an eNode B of the EvolvedUTRAN and a gateway of the core network. The eNode B has a similarfunction, although not exactly the same, to the eNode B and RNC of theexisting UMTS system, and the gateway has a function similar to theSGSN/GGSN of the existing system.

Another distinctive characteristic is that the control plane and theuser plane between the access system and the core network are exchangedwith different interfaces. While one Iu interface exists between the RNCand the SGSN in the existing UMTS system, two separate interfaces, i.e.,S1-MME and S1-U, are used in the Evolved Packet Core (SAE) system sincethe Mobility Management Entity (MME) 51 taking charge of the processingof a control signal is structured to be separated from the gateway (GW).For the GW, there are two types of gateways, a serving gateway(hereinafter, ‘S-GW’) 52 and a packet data network gateway (hereinafter,‘PDN-GW’ or ‘P-GW’) 53.

FIG. 2 is a view illustrating a relation between an (e)NodeB and a Home(e)NodeB.

In the 3rd or 4th generation mobile communication systems, attemptscontinue to increase their cell capacity in order to supporthigh-capacity and bi-directional services such as multimedia contents,streaming, and the like.

In other words, with the development of communication and widespreadmultimedia technologies, various high-capacity transmission technologiesare required, and accordingly, a method of allocating more frequencyresources is used to increase radio capacity, but there is a limit toallocate more frequency resources to a plurality of users withrestricted frequency resources.

In order to increase cell capacity, there has been an approach in whichhigh frequency bandwidth is used and the cell diameter is reduced. If acell having a small cell radius such as a pico cell is applied, it ispossible to use a higher bandwidth than the frequency that has been usedin the existing cellular system, thereby providing an advantage capableof transmitting more information. However, more base stations should beinstalled in the same area, thereby having a disadvantage of highinvestment cost.

In recent years, a femto base station such as Home (e)NodeB 30 has beenproposed among the approaches for increasing cell capacity using such asmall cell.

Studies on the Home (e)NodeB 30 have been started by 3GPP Home (e)NodeBWG3, and also in recent years, actively proceeded by SA WG.

An (e)NodeB 20 illustrated in FIG. 2 corresponds to a macro basestation, and a Home (e)NodeB 30 illustrated FIG. 2 may be a femto basestation. This specification will be described based on the terms of3GPP, and the term (e)NodeB is used when referring to both NodeB andeNodeB. Also, the term Home (e)NodeB is used when referring to both HomeNodeB and Home eNodeB.

The (e)NodeB 20 transmits and receives a signal of the MME 51 andcontrol plane, and transmits or receives a signal of the S-GW 52 anduser plane. Similarly, the Home (e)NodeB 30 also transmits or receives asignal of the MME 51 and control plane, and transmits and receives dataof the S-GW 52 and user plane. The PDN-GW 53 performs a role oftransmitting and/or receiving data from the S-GW 52 to and/or from an IPservice network of the mobile communication service provider.

The interface illustrated in a dotted line denotes a control signaltransmission between an (e)NodeB 20 and a Home (e)NodeB 30 and MME 510.Also, the interface illustrated in a solid line denotes a datatransmission of the user plane.

FIG. 3 is an exemplary view illustrating a network structure including aHome Node and a Home (e)NodeB.

As illustrated in FIG. 3( a), a core network 50 may include a MME 51, aserving gateway (S-GW) 52, a SGSN 56, and a packet data network gatewayor PDN gateway (P-GW) 53. In addition, the core network 50 may furtherinclude a PCRF 54, and HSS 55.

In FIG. 3( a), there are illustrated a Home NodeB 31 by the UMTSterrestrial radio access network (UTRAN) and a Home eNodeB 32 by theevolved-UTRAN (E-UTRAN) 32. The Home NodeB 31 by the UTRAN is connectedto the SGSN 56 through a gateway 35. The Home eNodeB 32 by the E-UTRANis connected to the MME 51 and the S-GW 52. Here, a control signal istransmitted to the MME 51, and a user data signal is transmitted to theS-GW 52. Furthermore, there may exist a gateway 35 between the HomeeNodeB 32 by the E-UTRAN and the MME 51.

On the other hand, referring to FIG. 3( b), there is illustrated aninterface of the Home eNodeB 32 by the E-UTRAN. The Home eNodeB 32 bythe E-UTRAN and the gateway 35 are referred to as a Home eNodeBsubsystem. The Home eNodeB 32 by the E-UTRAN is connected to the UE 10through an LTE-Uu interface. The Home eNodeB 32 by the E-UTRAN isconnected to the MME 51 through a S1-MME interface. Also, the HomeeNodeB 32 by the E-UTRAN is connected to the S-GW 52 through a S1-Uinterface. Here, the S1-MME interface and the S1-U interface may passthrough the gateway 35. The MME 51 and the S-GW 52 are connected to eachother through a S11 interface. Furthermore, the MME 51 and the HSS 55are connected to each other through a S6a interface.

FIG. 4 is an exemplary view illustrating an interface between a Home(e)NodeB 32 and a MME 51 illustrated in FIG. 3 using a protocol stack.

As illustrated in FIG. 4, the Home eNodeB 32 and the MME 51 may includea first layer (physical layer), a second layer (medium access controllayer), a third layer (Internet protocol (IP) layer), a signalingcontrol transmission protocol (SCTP), and a S1 application protocol(S1-AP), respectively.

The S1-AP is an application layer protocol between the Home eNodeB 32and the

MME 51. The SCTP guarantees the transmission of a signaling messagebetween the Home eNodeB 32 and the MME 51.

DISCLOSURE OF INVENTION Solution to Problem

In the foregoing related art, the UE 10 can select an appropriate cellbetween a cell of the (e)NodeB 20 and a cell of the Home (e)NodeB 30 toperform a cell selection operation. Furthermore, the UE 10 may access orattach to the selected cell.

However, there is a problem that the UE 10 cannot perform a handover tothe (e)NodeB 20 while receiving a service through the Home (e)NodeB 30subsequent to accessing or attaching to either one of the cells, forinstance, Home (e)NodeB 30.

Particularly, since the handover cannot be performed, there is a problemthat the ongoing service of the UE 10 is dropped when encountering asituation that the Home (e)NodeB 30 cannot provide a service to the UE10 any more.

In order to solve the foregoing problem, there have been attempts tohand over the

UE 10 receiving a service through the Home (e)NodeB 30 to the (e)NodeB20.

However, those attempts have merely handed over to the (e)NodeB 20 if itis more appropriate to receive a service from the (e)NodeB 20 as the UE10 receiving a service through the Home (e)NodeB 30 is geographicallymoved.

Nevertheless, there may be a situation that the Home (e)NodeB 30 cannottemporarily provide a service according to various reasons, but thosevarious reasons have not been considered in the related art. As aresult, there is a problem that the on-going service of the UE 10 isdropped due to the situation of the Home (e)NodeB 30.

Accordingly, an object of the present invention is to solve theforegoing problem. In other words, it is an object of the presentinvention to hand over the UE 10 receiving a service through aparticular Home (e)NodeB to another appropriate Home (e)NodeB or(e)NodeB according to the condition of the Home (e)NodeB.

Furthermore, another object of the present invention is to perform areverse handover for the UE, which performs a handover to the anotherappropriate Home (e)NodeB or (e)NodeB, to the particular Home (e)NodeBwhen the particular Home (e)NodeB is changed to a condition capable ofproviding a service.

In order to accomplish the foregoing object, according to the presentinvention, in case where it is inappropriate for a particular Home(e)NodeB providing a service to a particular UE to continue to providethe service on the ground of a changed status, such as overload,temporary drop due to maintenance, expiry of CSG membership for the UE,increase of temporary interference, policy change, or the like, the Home(e)NodeB is allowed to transmit information on a cause that the UEperforms a handover to another base station to an control entity withina network.

Furthermore, according to the present invention, it is allowed for theentity within the network to store information on the cause. Inaddition, according to the present invention, if the status of theparticular Home (e)NodeB is changed, then the entity within the networkis allowed to perform a reverse handover for the UE to the particularHome (e)NodeB on the basis of the information on the changed status andthe information on the cause of the handover.

Specifically, in order to accomplish the foregoing object, according tothe present invention, there is provided a method of controllinghandover in a server in charge of a control plane within a mobilecommunication network. The handover control method may include receivinga handover request message including information on a cause that theterminal performs a handover from a Home (e)NodeB corresponding to aserving base station of the terminal to another base station;transmitting a handover command message to the Home (e)NodeB for theterminal to perform a handover to another base station; receiving astatus report message from the Home (e)NodeB; determining whether or notthe terminal performs a reverse handover from the another base stationto the Home (e)NodeB based on whether the cause has been solved if thestatus report message is received; and transmitting a control message toperform the reverse handover based on the determination.

In order to accomplish the foregoing object, according to the presentinvention, there is provided a server in charge of a control planewithin a mobile communication network. The server may include atransceiver transmitting a handover command message to a Home (e)NodeBfor a terminal to perform a handover to another base station whenreceiving a handover request message including information on a causethat the terminal performs a handover from the Home (e)NodeBcorresponding to a serving base station of the terminal to the anotherbase station; and a controller determining whether or not the terminalperforms a reverse handover from the another base station to the Home(e)NodeB based on whether the cause has been solved if a status reportmessage from the Home (e)NodeB through the transceiver is received.

The handover request message may further include at least one ofidentification information of the terminal and information on theanother base station.

The cause may include at least one of overload, maintenance, change ofthe CSG membership for the Home (e)NodeB, and policy change.

The status report message may be received from the Home (e)NodeB whenthe cause of the handover has been changed. The status report messagemay include information on whether the cause is solved.

The status report message may include at least one of a parameterindicating that the cause has been solved, information on the changedstatus, and information on the solved cause.

In the determining step, at least one of subscriber information of theterminal, policy information, membership information of the terminal forthe Home (e)NodeB may be considered.

The control message may be transmitted to the another base station, andcorrespond to a message indicating reverse handover, a reverse handoverrequest message or a handover request message.

The control message may be transmitted to the Home (e)NodeB, andcorrespond to a handover request message.

The information stored within the handover request message may bestored.

On the other hand, in order to accomplish the foregoing object, there isprovided a method of handing over a terminal in a Home (e)NodeB systemcorresponding to a serving base station of the terminal. The handovermethod may include transmitting a handover request message includinginformation on a cause that the terminal performs a handover to anotherbase station to an entity within a network; receiving a handover commandmessage from the entity within the network; delivering the handovercommand message to the terminal; and transmitting a status reportmessage to the entity within the network when a situation correspondingto cause is changed.

The present invention is provided to solve the foregoing problem. Inother words, according to the present invention, user equipment (UE)receiving a service from a particular Home (e)NodeBperforms a handoverto another appropriate Home (e)NodeB or (e)NodeB based on the status ofthe Home (e)NodeB. Furthermore, according to the present invention, whenthe status of the particular Home (e)NodeB is changed to a state capableof providing a service, the UE that has been performs handover to theanother appropriate Home (e)NodeB or (e)NodeB performs a reversehandover to the particular Home (e)NodeB.

As a result, the present invention allows the UE to receive a servicethrough a Home (e)NodeB capable of providing faster service. Inaddition, the present invention reduces the load of another basestation, for instance, (e)NodeB.

Moreover, the present invention allows the UE to be connected to aparticular H(e)NB even though radio signal in a cell of the neighboringbase station is stronger, thereby allowing the UE to receive a moreeffective service.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a structural diagram illustrating an evolved mobilecommunication network;

FIG. 2 is a view illustrating a relation between an (e)NodeB and a Home(e)NodeB;

FIG. 3 is an exemplary view illustrating a network structure including aHome Node and a Home (e)NodeB;

FIG. 4 is an exemplary view illustrating an interface between a Home(e)NodeB 32 and a MME 51 illustrated in FIG. 3 using a protocol stack;

FIG. 5 is conceptual diagrams illustrating the access modes of a Home(e)NodeB 300;

FIG. 6 is a flow chart illustrating an example in which UE 100 performshandover from a Home (e)NodeB 300 to an (e)NodeB;

FIG. 7 is a flow chart illustrating an example in which the handed-overUE as illustrated in FIG. 6 performs a handover again to Home (e)NodeB300;

FIG. 8 is a flow chart illustrating the operation of a MME 510illustrated in FIGS. 6 and 7;

FIG. 9 is a flow chart illustrating a Home (e)NodeB 300 illustrated inFIGS. 6 and 7; and

FIG. 10 is a configuration block diagram of a Home (e)NodeB 300 and aMME 510.

MODE FOR INVENTION

The present invention will be described on the basis of a universalmobile telecommunication system (UMTS) and an evolved packet core (EPC).However, the present invention is not limited to such communicationsystems, and it may be also applicable to all kinds of communicationsystems and methods to which the technical spirit of the presentinvention is applied.

It should be noted that technological terms used herein are merely usedto describe a specific embodiment, but not to limit the presentinvention. Also, unless particularly defined otherwise, technologicalterms used herein should be construed as a meaning that is generallyunderstood by those having ordinary skill in the art to which theinvention pertains, and should not be construed too broadly or toonarrowly. Furthermore, if technological terms used herein are wrongterms unable to correctly express the spirit of the invention, then theyshould be replaced by technological terms that are properly understoodby those skilled in the art. In addition, general terms used in thisinvention should be construed based on the definition of dictionary, orthe context, and should not be construed too broadly or too narrowly.

Incidentally, unless clearly used otherwise, expressions in the singularnumber include a plural meaning. In this application, the terms“comprising” and “including” should not be construed to necessarilyinclude all of the elements or steps disclosed herein, and should beconstrued not to include some of the elements or steps thereof, orshould be construed to further include additional elements or steps.

The terms used herein including an ordinal number such as first, second,etc. can be used to describe various elements, but the elements shouldnot be limited by those terms. The terms are used merely to distinguishan element from the other element. For example, a first element may benamed to a second element, and similarly, a second element may be namedto a first element.

In case where an element is “connected” or “linked” to the otherelement, it may be directly connected or linked to the other element,but another element may be existed therebetween. On the contrary, incase where an element is “directly connected” or “directly linked” toanother element, it should be understood that any other element is notexisted therebetween.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings, and thesame or similar elements are designated with the same numeral referencesregardless of the numerals in the drawings and their redundantdescription will be omitted. In describing the present invention,moreover, the detailed description will be omitted when a specificdescription for publicly known technologies to which the inventionpertains is judged to obscure the gist of the present invention. Also,it should be noted that the accompanying drawings are merely illustratedto easily explain the spirit of the invention, and therefore, theyshould not be construed to limit the spirit of the invention by theaccompanying drawings. The spirit of the invention should be construedas being extended even to all changes, equivalents, and substitutesother than the accompanying drawings. Hereinafter, the term “terminal”is used, but the terminal may be also called UE (User Equipment), ME(Mobile Equipment), and MS (Mobile Station). Furthermore, the terminalmay be portable equipment such as a portable phone, a PDA, a smartphone, and a notebook, or non-portable equipment such as a PC, and avehicle-loaded device.

Furthermore, the term “Home (e)NodeB” is used below, but the Home(e)NodeB may be called a femto base station, a Home NodeB, and a HomeeNodeB.

Definition of Terms

Hereinafter, the terms used in this specification will be brieflydefined prior to describing with reference to the drawings.

UMTS: It is an abbreviation of Universal Mobile Telecommunication Systemand denotes the 3rd mobile communication network.

EPS: It is an abbreviation of Evolved Packet System, and denotes a corenetwork supporting a Long Term Evolution (LTE) network. It is a networkin the form of an evolved UMTS.

NodeB: It is installed outdoors as a base station of the UMTS network,and the size of the cell coverage corresponds to a macro cell.

eNodeB: It is installed outdoors as a base station of the Evolved PacketCore (EPC) network, and the size of the cell coverage corresponds to amacro cell.

(e)NodeB: It is a term indicating both NodeB and eNodeB.

Home NodeB: It is installed indoors as a base station of the UMTSnetwork, and the size of the cell coverage corresponds to a femto cell.

Home eNodeB: It is installed indoors as a base station of the EPSnetwork, and the size of the cell coverage corresponds to a femto cell.

Home (e)NodeB: It is a term indicating both Home NodeB and Home eNodeB.

Home (e)NodeB Gateway: It is connected to at least one Home (e)NodeB asa gateway performing a role of interfacing with a core network.

Home (e)NodeB Subsystem: It is a form of combining a Home (e)NodeB witha Home (e)NodeB Gateway into a set to manage a radio network. Both theHome (e)NodeB subsystem and the Home (e)NodeB perform a role of managinga radio network to link with a core network, and thus considered as anaggregate form. Accordingly, the terms “Home (e)NodeB” and “Home(e)NodeB subsystem” are interchangeably used below.

Closed Subscriber Group (CSG): It denotes a group having at least oneHome (e)NodeB. The Home (e)NodeBs belonging to the CSG have a same CSGID. Each user receives a license for each CSG.

Closed Access Mode: It denotes a mode in which a Home (e)NodeB isoperated as a CSG cell. It operates in a method of allowing an accessonly to a user terminal that is allowed for the relevant cell. In otherwords, a terminal having an authority for the particular CSG IDssupported by a Home (e)NodeB is only accessible.

Open Access Mode: It denotes a mode in which a Home (e)NodeB is operatedin a method similar to a normal cell (non-CSG cell) without the conceptof CSG. In other words, it is operated like a normal (e)NodeB.

Hybrid Access Mode: It denotes a mode in which a Home (e)NodeB isoperated as a CSG cell, but its access is also allowed to an non-CSGsubscriber. The access is allowed for a user terminal having aparticular CSG ID that can be supported by the relevant cell to providea Home (e)NodeB service, and operated in a method in which even aterminal having no CSG authority is allowed to access.

CSG Cell: As a cell in part of a PLMN, it is accessible only by a memberof the CSG group. For the purpose of this, the CSG cell broadcasts a CSGID. The CSG cells sharing the same ID may be identified or managed as agroup for the mobile management and charging.

CSG ID: As an identifier broadcasted by the CSG cell, it is used tofacilitate the access of an authorized member of the CSG. The CSG ID isunique only within a PLMN.

Tracking Area: It is a basic unit for tracking the location of the UE100 being operated in an idle mode. In other words, it is a basic unitfor the location registration of the UE. The ID of the particular areais referred to as a tracking area identity (TAI). In the EPS, a terminalcan be simultaneously location-registered to several TAs, and a TM listis managed at this time.

TAI LIST: It is a set of location information, i.e., (e)NodeB or Home(e)NodeB in which the location registration is performed on a HSS or HLRthrough a MME based on the location movement of the UE.

Policy and Charging Control (PC): It denotes a management for theservice provider's policy and charging.

Policy and Charging Rule Function (PCRF): It is a functional networkelement having the service provider's policy and charging rules toperform the PCC, and performs a role of providing the service provider'spolicy and charging rules to other nodes.

Description for the Concept of Schemes Proposed by This Specification

According to this specification, a scheme is proposed that in case whereit is inappropriate for a particular Home (e)NodeB providing a serviceto a particular UE to continue to provide the service on the ground of achanged status, such as overload, temporary drop due to maintenance,expiry of CSG membership for the UE, increase of temporary interference,policy change, or the like, the UE performs a handover to anotherappropriate Home (e)NodeB or (e)NodeB by a control entity within anetwork, for instance, MME.

Furthermore, according to this specification, a scheme is proposed thatthe entity within the network stores information on a cause that the UEperforms handover from the particular Home (e)NodeB to the anotherappropriate Home (e)NodeB or (e)NodeB. On the other hand, if the statusof the particular Home (e)NodeB is changed, then the entity within thenetwork performs a reverse handover for the UE to the particular Home(e)NodeB on the basis of the information on the changed status and theinformation on the cause of the handover.

Hereinafter, it will be described in detail with reference to thedrawings.

FIG. 5 is conceptual diagrams illustrating the access modes of a Home(e)NodeB 300.

A femto base station illustrated in the drawing, for instance, Home(e)NodeB 300, can be operated in three modes, such as an open accessmode of FIG. 5( a), a closed access mode of FIG. 5( b), and a hybridaccess mode of FIG. 5( c). The closed access mode can be used only bythe CSG members supported by its own group. The open access mode is amode that is operated like a normal access without the concept of CSG,and in this case, a normal UE is also accessible. The hybrid access modeis a mode in which the Home (e)NodeB 300 is operated in a closed accessmode or open access mode.

First, referring to FIG. 5( a), there is illustrated an example of theopen access mode. As described in the forgoing definition of terms, theopen access mode denotes a situation in which a cell of the Home(e)NodeB 300 is accessible by all UEs without restricting the access.Accordingly, the access by a plurality of UEs 110, 120, 130 is allallowed.

Next, referring to FIG. 5( b), there is illustrated an example of theclosed access mode. In the closed access mode, a Home (e)NodeB 300 isoperated as a cell of the closed subscriber group (CSG). In other words,the Home (e)NodeB 300 allows the access only for a user terminal allowedthereto. The Home (e)NodeBs 300 have CSG IDs capable of providing aservice. For the purpose of this, the Home (e)NodeB 300 stores an IDlist of the closed subscriber group (CSG). Each user receives a licensefor each CSG. For example, the Home (e)NodeB 300 illustrated in thedrawing supports a first closed subscriber group (CSG), a second closedsubscriber group (CSG), and a third closed subscriber group (CSG). Here,if the first UE 110 does not have memberships of the first through thethird closed subscriber groups but the second UE 120 has thememberships, then the Home (e)NodeB 300 rejects an access of the firstUE 110, and authorizes only an access of the second UE 120 asillustrated in the drawing.

On the other hand, the CSG membership may have an expired time. Whenreceiving an access request from the second UE 120 after the expiredtimed has been expired, the Home (e)NodeB 300 may reject the accessrequest.

However, when the CSG membership of the UE for the Home (e)NodeB 300 isexpired while the Home (e)NodeB 300 provides a service to the second UE120, the service being provided to the UE is suspended by the Home(e)NodeB 300 in the prior art. However, according to this specification,if the membership is expired, then the UE 100 may perform handover toanother appropriate Home (e)NodeB or (e)NodeB.

The Home (e)NodeB 300 may provide information (or parameter) on such acause of the handover, i.e., information on the expiry of the CSGmembership, to the network entity, for instance, MME. Then, the networkentity, for instance, MME, may store the information. Then, the networkentity, for instance, MME, may reversely hand over the UE to the Home(e)NodeB 300 when the membership is updated afterwards, based on theinformation.

Last, referring to FIG. 5( c), there is illustrated an example of thehybrid access mode. In the hybrid access mode, the Home (e)NodeB 300supports both a terminal of the closed subscriber group (CSG) and aterminal of the non-closed subscriber group (non-CSG).

In other words, the hybrid access mode is operated like a CSG cell forproviding a service to the closed subscriber group (CSG), but the accessof a terminal of the non-closed subscriber group (non-CSG) is allowed.For example, as illustrated in the drawing, the first UE 110 can accessto the Home (e)NodeB 300 even though it does not have a CSG membershipfor the Home (e)NodeB 300 (i.e., non-closed subscriber group (non-CSG)member). However, service quality of the first UE 110 corresponding tothe non-closed subscriber group (non-CSG) may be lower than that of theother UE having a CSG membership.

On the other hand, when the CSG membership of the second UE 120 for theHome (e)NodeB 300 is expired while the Home (e)NodeB 300 provides aservice to the UE, the second UE 120 may be switched over to annon-closed subscriber group (non-CSG) member to continue to receive theservice.

However, the quality is reduced as being switched to the non-closedsubscriber group (non-CSG). At this time, in case where it isinappropriate to continue to perform the service due to the reducedquality, the UE 100 may perform a handover to another appropriate Home(e)NodeB or (e)NodeB according to this specification.

The Home (e)NodeB 30 may provide information (or parameter) on the causeof the handover, for example, information on the switchover to thenon-closed subscriber group (non-CSG) member and dissatisfaction of thequality, to the network entity, for instance, MME. Then, the networkentity, for instance, MME, may store the information. Then, the networkentity, for instance, MME, may reversely hand over the UE to the Home(e)NodeB 300 when the membership is updated afterwards, based on theinformation.

FIG. 6 is a flow chart illustrating an example in which UE 100 performsa handover from a Home (e)NodeB 300 to an (e)NodeB.

Referring to FIG. 6, according to the present invention, in case wherethe UE 100 receiving a service from the Home (e)NodeB 300 performs ahandover to another base station, for instance, (e)NodeB 200 or Home(e)NodeB, a MME 510 stores information on the cause of the handover. Thetiming of storing the information may be various as illustrated in thedrawing. More specifically, it will be described below.

1) The UE 100 establishes a bearer with respect to a S-GW 520 and a P-GW530 in the user plane through the Home (e)NodeB 300 to transmit orreceive data in the user plane.

2) Subsequently, in case where it is inappropriate for the Home (e)NodeB300 to continue to provide the service on the ground of a changedstatus, for instance, overload, temporary drop due to maintenance,expiry of CSG membership for the UE, increase of temporary interference,policy change, or the like, the Home (e)NodeB 300 transmits a messageindicating that handover is required, or handover request message, forinstance, HO request message. The message indicating that handover isrequired, or handover request message is a message transmitted from asource base station, i.e., the Home (e)NodeB 300 illustrated in thedrawing, to a target base station, i.e., the (e)NodeB 200 illustrated inthe drawing. The message indicating that handover is required, orhandover request message is a message based on a S1 application protocol(S1-AP).

The message indicating that handover is required, or handover requestmessage may include information on an identifier capable of identifyingthe UE 100, i.e., ID (for example, UE S1AP ID) or parameter indicatingan identifier, and information on the handover cause (for instance,overload, etc. as described above) (or “Reduce load in serving cell”) orparameter indicating the cause. Furthermore, the message indicating thathandover is required, or handover request message may further include atleast one of identification information (for instance target eNodeBIdentity) of a target indicating a target TAI.

The message indicating that handover is required, or handover requestmessage may include at least one of parameters as illustrated in thefollowing table.

TABLE 1 Parameter Description Message Type Message Type MME UE S1AP ID Aparameter indicating information for a MME to identify a particular UE.Aparameter used for distinguishing a bearer established between theMMEand the UE. eNB UE S1AP ID A parameter indicating information for aneNodeB to identify a particular UE.A parameter used for distinguishing abearer established between the eNodeBand the UE. Handover Type Indicatethe type of handover Cause A parameter indicating a cause that thismessage has been started. The cause of handover may include overload,Reduce load in serving cell, temporary drop due to maintenance, expiryof CSG membership for the UE, increase of temporary in- terference,policy change, or the like. Target ID ID of a target base station

3) If the message indicating that handover is required, or handoverrequest message is received, then the MME 510 transmits for instance, aHO request message, for instance, HO request to the target base station,for instance, (e)NodeB 200. At this time, the MME 510 can determinewhether or not the handover is required based on the cause included inthe message. The handover request message allows the target basestation, i.e., (e)NodeB 200, to generate a context (i.e., UE context)including bearer-related information. The handover request message mayinclude information on the cause of handover or a parameter indicatingthe cause.

During the process, the MME 510 may store at least one of the parameterswithin a message indicating that the handover is required, or handoverrequest message. For example, the MME 510 stores information on thecause of handover or a parameter indicating the cause (i.e., the Causeparameter), and identification information of a target base station or aparameter indicating an identifier (i.e., Target ID parameter). Thestoring may be performed within a context with respect to the UE 100,for instance, MME bearer context.

On the other hand, the MME 510 may acquire information on the cause ofhandover or a parameter indicating the cause (i.e., the Causeparameter), and identification information of a target base station or aparameter indicating an identifier from another entity within a network.The acquiring may not be performed during the foregoing process but maybe performed during any process, for example, the process, or the like.Also, the storing may not be performed during the foregoing process butmay be performed, for example, during the 4th or 9th process amongvarious processes as will be described later.

4) The target base station, i.e., (e)NodeB 200, transmits a handoverrequest acknowledgement (response) message, for instance, HO request ACKmessage, in response to the handover request message. The handoverrequest acknowledgement (response) message may include TEID information.

When the parameters are not stored during the foregoing 3rd process, atleast one of the parameters stored within the message indicating thatthe handover is required, or handover request message may be extractedand stored during the foregoing process.

5) If the handover request acknowledgement (response) message isreceived, then the

MME 510 transmits a handover command message, for instance, HO commandmessage to the Home (e)NodeB 300 in order to perform a handover of theUE during the radio section. Then, the Home (e)NodeB 300 delivers thehandover command message to the UE 100. The handover command message mayinclude a parameter indicating identification information or identifierof the target base station, i.e., (e)NodeB 200.

6) If the handover command message is received, the UE 100 transmits ahandover confirmation (approval) message, for instance, HO Confirmmessage to the target base station, i.e., (e)NodeB 200.

7) In case where there exists downlink data to be transmitted when thehandover during the radio section has been completed as described above,the target base station, i.e., (e)NodeB 200 transmits the downlink datato the UE 100.

8) Similarly, in case where there exists uplink data, the UE 100 alsotransmits the uplink data to the (e)NodeB 200, and then the (e)NodeB 200delivers the uplink data to the S-GW 520.

9) On the other hand, the (e)NodeB 200 transmits a message notifyingthat the carried-out handover has been recognized, or a messageindicating that handover has been completed, for instance, HO notifymessage, to the MME 510.

In case where the parameters are not stored during the foregoing 3rd or4th process, then the MME 510 may extract and store a message indicatingthat the handover is required, or at least one of the parameters thathave been within the handover request message during the process. Theadvantage of when storing during the foregoing process is as follows. Ifthe parameters are stored during the foregoing 3rd or 4th process butthe handover is failed during the fifth process, the stored parametersshould be abandoned, and thus it may be unnecessary storing.Accordingly, it may be more effective to store the parameters during the9th process.

10) If a message indicating that the handover has been completed isreceived, the

MME 510 transmits a bearer update (modify) message, for instance, Modifybearer request message, to the S-GW 520 in order to update a context(for instance, bearer context). The S-GW 520 delivers it to the P-GW530. The bearer update (modify) message may include a target basestation, i.e., an address of the (e)NodeB 200, and TEID in order totransmit downlink data to the UE 100.

11) Then, the P-GW 530 transmits the bearer update (modify) message tothe S-GW 520, and the S-GW 520 transmits the bearer update (modify)message to the MME 510.

In case where the parameters are not stored during the foregoing 3rd,4th, or 9th process, the MME 510 may extract and store a messageindicating that the handover is required, or at least one of theparameters that have been within the handover request message during theprocess.

12) Then, the downlink data to the UE 100 may be delivered to the UE 100through the (e)NodeB 200.

13) On the other hand, a tracking area update (TAU) process for HSSlocation registration of the UE 100 is performed.

14) The MME 510 transmits a context for the UE that remained in thesource base station, i.e., Home (e)NodeB 300, namely, a context releasemessage, for instance, UE Context release command message, to the Home(e)NodeB 300 in order to release the UE context

15) The Home (e)NodeB 300 transmits a response message, for instance, UEContext release complete message, to the MME 510.

16) On the other hand, the UE 100 transmits and/or receives data in theuser plane to and/or from the S-GW/P-GW 520/530 through the (e)NodeB200.

As described above, the MME 510 can obtain information or parameter fora reason that the UE 100 performs a handover to another base station,and store the information or parameter. The storing may be performedduring any process illustrated in FIG. 6.

FIG. 7 is a flow chart illustrating an example in which the handed-overUE as illustrated in FIG. 6 performs a reverse handover again to Home(e)NodeB 300.

Referring to FIG. 7, when the status of the original base station of theUE 100, i.e., Home (e)NodeB 300, is changed, in other words, when thecause of the handover is solved (removal of overload, completion ofmaintenance, update of CSG membership for the UE, decrease of temporaryinterference, policy change), the MME 510 is allowed to perform areverse handover for the UE 100 to the Home (e)NodeB 300 on the basis ofthe information on the changed status and the information on the causeof the handover. More specifically, it will be described below.

The process numbers as illustrated in FIG. 7 are given subsequent to the16th process of FIG. 6.

16) As illustrated in FIG. 6, the UE 100 transmits and/or receives datain the user plane from and/or to the S-GW/P-GW 520/530 through the(e)NodeB 200 after performing a handover from the Home (e)NodeB 300 tothe (e)NodeB 200.

17) When the status of the Home (e)NodeB 300 corresponding to theoriginal base station of the UE 100 is changed prior to the handover, inother words, when the cause of the handover is solved (removal ofoverload, completion of maintenance, update of CSG membership for theUE, decrease of temporary interference, policy change), the Home(e)NodeB 300 transmits a status report message, for instance, StatusReport message, to the MME 510. The status report message may include aparameter indicating the cause of handover has been solved. Also, thestatus report message may include information on the changed status orinformation of the solved cause. The information on the solved cause mayindicate that resource shortage phenomenon has been solved, forinstance, as illustrated in the drawing. Also, the information on thesolved cause may be an update of the CSG membership.

18) The MME 510 searches whether or not there exists a terminal that hasperformed a handover from the Home (e)NodeB 300 to another base station.At this time, the search is performed with respect to a context, forinstance, UE Context. Through the search, the MME 510 confirms that theterminal that has performed a handover from the Home (e)NodeB 300 toanother base station is the UE 100.

Then, the MME 510 may obtain at least one of subscriber information ofthe UE 100, information on the policy of the Home (e)NodeB, policyinformation (for instance, policy of the (e)NodeB, policy of the Home(e)NodeB), and information on the CSG membership of the UE for the Home(e)NodeB 300 from a HSS 550. The subscriber information of the UE 100may have been delivered to the MME through an interaction with the HSSduring the previous attach/TAU process, and therefore, the MME 510 mayalready have the information.

19) The MME 510 determines whether to perform a reverse handover for theUE 100 to the Home (e)NodeB 300 corresponding to the original basestation on the basis of the status report message and the obtainedinformation. At this time, the MME 510 may consider at least one of theobtained subscriber information, policy of the service provider, policyof the Home (e)NodeB, and information on the CSG membership in thedetermining step. If the UE 100 is a temporary member, then the CSGinformation obtained from the HSS is CSG ID and time period, andtherefore, the MME 510 may use the time period as a basis ofdetermination. Also, the CSG information is based on subscriberinformation, and therefore, it may be possible to use the information asa basis of selection according to the type of CSG member or priority ofthe type, or according to the priority of a group to which the memberbelongs.

As an example for the determination, in case where it is grasped thatthe solved cause is an update of the CSG membership based on the statusreport message, the MME 510 may determine that the UE performs ahandover to the Home (e)NodeB 300 based on the updated membership.Furthermore, in case where it is grasped that the solved caused is aremoval of resource shortage phenomenon or when the resource of the Home(e)NodeB is larger than the resource of (e)NodeB corresponding to acurrent serving cell based on the status report message, the MME 510 maydetermine that the UE 100 performs a handover to the Home (e)NodeB 300.In addition, in case where it is grasped that the solved caused is aremoval of resource shortage phenomenon or in case where the resource ofthe Home (e)NodeB is sufficiently large at any normal time based on thestatus report message, the MME 510 may determine that the UE 100performs a reverse handover to the Home (e)NodeB 300.

On the other hand, when information on the solved cause is not includedwithin the status report message during the 17th process, the MME 510may grasp information on the solved cause from another entity within anetwork based on the status report message. For example, if the statusreport message is received, then the MME 510 checks information on thestored handover cause. Then, the MME 510 may inquire of an appropriatenetwork entity about a current status of the Home (e)NodeB 300 based onthe information on the handover cause. As a specific example, in casewhere it is confirmed that the handover cause was an expiry of the CSGmembership, the MME 510 may check with the HSS 550 whether or not themembership of the UE 100 for the Home (e)NodeB 300 has been updated.

20) In case where it is determined to perform a reverse handover asdescribed above, the MME 510 transmits a message indicating reversehandover, or reverse handover request message, for instance, Reverse HOrequest message to the (e)NodeB 200. The reverse handover requestmessage may include identification information of a target base station(i.e., Home (e)NodeB 300) and a parameter indicating the identificationinformation of the UE (for instance, S1 AP ID parameter). The reversehandover request message, for instance, Reverse HO request message, is anewly defined message according to the present invention. The messageindicating a reverse handover may be typically provided by adding afield indicating a reverse handover in a message used for indicating ahandover, for instance, HO command message.

21-23) Then, the (e)NodeB 200 transmits a message indicating thathandover is required, or handover request message, for instance, HOrequest message to the MME 510 as illustrated in the 2nd process of FIG.6. Here, the 22nd and 23rd processes illustrated in the drawing aresimilar to the 3rd and 4th processes of FIG. 6, respectively. On theother hand, the 21st through 23rd processes may be omitted according tothe implementation of the reverse handover request message during the20th process.

24-28) The 24th through 28th processes illustrated in the drawing aresimilar to the 5th, 6th, 9th, 10th, and 11th processes of FIG. 6, andthus they will not be described in detail herein.

29) A tracking area update (TAU) process for HSS location registrationof the UE 100 is performed. At this time, when the Home (e)NodeB 300 isoperated in a closed access mode, an access control process for checkingwhether or not the UE 100 has a CSG membership for the Home (e)NodeB 300may be performed. The access control process denotes a process in whichthe MME 510 determines whether to receive a request of the relevant UEbased on the subscriber information when receiving a TAU request fromthe UE 100 through the Home (e)NodeB 300. In other words, it means aprocess in which the MME 510 checks information such as the CSG ID todetermine whether to allow an access to the relevant UE.

30-32) The 30th through 32nd processes are similar to the description ofFIG. 6, and thus will be omitted herein.

FIG. 8 is a flow chart illustrating the operation of a MME 510illustrated in FIGS. 6 and 7.

Referring to FIG. 8, the MME 510 receives a handover request messageincluding information or a parameter on a cause that a terminal shouldperform a handover from a Home (e)NodeB corresponding to a serving basestation of the terminal to another base station (S110).

Then, the information or parameter on the cause within the handoverrequest message is stored (S120).

Then, the MME 510 hands over the UE 100 to another base station (S130).In other words, the MME 510 transmits a handover command message to theHome (e)NodeB so that the terminal performs a handover to another basestation.

Subsequently, if a status report message is received from the Home(e)NodeB, then the MME 510 determines whether the UE 100 is to perform areverse handover to the Home (e)NodeB 300 (S150). In other words, whenreceiving the status report message, the MME 510 determines whether theterminal is to perform a revere handover from the another base stationto the Home (e)NodeB based on whether or not the cause has been solved.

If it is determined to perform the reverse handover, then the MME 510performs a reverse handover for the UE 100 (S160). In other words, theMME 510 transmits a control message to perform the reverse handoveraccording to the determination.

The storing process (S120) may be performed between the process of S110and the process of S130, or may be performed between the otherprocesses.

On the other hand, the MME 510 may further perform part or all of theprocesses illustrated in FIGS. 6 and 7 in addition to the processesillustrated in FIG. 8. Otherwise, the MME 510 may not perform part ofthe processes illustrated in FIGS. 6 and 7 to omit them.

FIG. 9 is a flow chart illustrating a Home (e)NodeB 300 illustrated inFIGS. 6 and 7.

Referring to FIG. 9, in case where the UE perform a handover to anotherbase station (S210), the Home (e)NodeB 300 transmits a handover requestmessage including information on a cause of that the UE performs ahandover to the another base station to the MME 510 (S220).

If a handover command from the MME (S230) is received, the Home (e)NodeB300 hands over the UE to another base station (S240). In other words,the Home (e)NodeB delivers the handover command message to the UE tohand over the UE.

On the other hand, if a situation corresponding to the cause of thehandover is changed (S250), then the Home (e)NodeB 300 transmits astatus report message to an entity within the network (S260).

On the other hand, the Home (e)NodeB 300 may further perform part or allof the processes illustrated in FIGS. 6 and 7 in addition to theprocesses illustrated in FIG. 8. Otherwise, Home (e)NodeB 300 may notperform part of the processes illustrated in FIGS. 6 and 7 to omit them.

As described up to this point, a method according to the presentinvention can be realized by software, hardware, or their combination.For example, a method according to the present invention may be storedin a storage medium (e.g., internal terminal, flash memory, hard disk,etc.), and may be realized by codes or commands within a softwareprogram that is executable by a processor, such as microprocessor,controller, microcontroller, application specific integrated circuit(ASIC). It will be described with reference to FIG. 10.

FIG. 10 is a configuration block diagram of a Home (e)NodeB 300 and aMME 510.

As illustrated in FIG. 10, the Home (e)NodeB 300 may include a storagemeans 301, a controller 302, and a transceiver 303.

The MME 510 may include a storage means 511, a controller 512, and atransceiver 513.

The storage means store a software program in which a method illustratedin FIGS. 5 through 9 is realized.

Each of the controllers controls the storage means and the transceiversrespectively. Specifically, the controllers implement each of theforegoing methods stored in the storage means respectively. Then, eachof the controllers transmits the foregoing signals through thetransceivers.

Specifically, the transceiver 303 of the Home (e)NodeB 300 transmits ahandover request message including information on a cause that the UEperforms a handover to another base station to the MME 510, and receivesa handover command message from the MME 510. Then, the transceiver 303delivers the handover command message to the UE 100.

If it is confirmed that a situation corresponding to the cause ischanged, then the controller 302 controls the transceiver 303 totransmit a status report message to the MME 510.

On the other hand, if a handover request message including informationon a cause that the UE 100 perform a handover to another base stationfrom the Home (e)NodeB is received, then the transceiver 513 of the MME510 transmits a handover command message to the Home (e)NodeB 300 tohand over the UE 100 to another base station.

If a status report message is received from the Home (e)NodeB 300through the transceiver 513, the controller 512 of the MME 510determines whether to reversely hand over to the Home (e)NodeB based onwhether or not the cause has been solved.

Though preferred embodiments of present invention are exemplarilydescribed as disclosed above, the scope of the invention is not limitedto those specific embodiments, and thus various modifications,variations, and improvements can be made to the present inventionwithout departing from the spirit of the invention, and within the scopeof the appended claims.

1. A method of supporting handover by a network node in a mobilecommunication system, the method comprising: receiving a message relatedto a handover, including a first parameter indicating a cause for thehandover; transmitting a handover command message to a first basestation for a terminal to perform a handover to a second base station,when the network node determines that handover is needed based on thecause; receiving a status report message including a second parameterindicating whether or not the cause for handover is solved; andtransmitting a reverse handover request message to the second basestation for the terminal to perform reverse handover to a target basestation, when the network node determines that a reverse handover isneeded, wherein the network node determines that the handover is needed,if the first base station is a closed subscriber group (CSG) cell andthe cause is an expiry of a CSG membership, and wherein the network nodedetermines that the reverse handover is needed, if the cause is anexpiry of a CSG membership and the second parameter indicates updatingof a CSG membership.
 2. The method of claim 1, wherein the causeincludes overload, maintenance, and a policy change.
 3. The method ofclaim 1, wherein the second parameter indicates at least one of removalof overload, completion of maintenance, decrease of temporaryinterference or a policy change.
 4. The method of claim 1, wherein ifthe first base station is operating in a hybrid access mode and thecause is an expiry of a CSG membership, a condition that a signalquality is under a predetermined threshold is satisfied to transmit thehandover command message.
 5. The method of claim 1, wherein the reversehandover message includes identification information of the target basestation.
 6. The method of claim 1, wherein the target base station isthe first base station.
 7. The method of claim 1, the method furthercomprising: transmitting a control message to perform the reversehandover to the target base station.
 8. A server in charge of a controlplane within a mobile communication network, the server comprising: atransceiver; and a controller configured to: control the transceiver toreceive a message related to a handover, including a first parameterindicating a cause for the handover, transmit a handover command messageto a first base station for a terminal to perform a handover to a secondbase station, when the network node determines that handover is neededbased on the cause, receive a status report message including a secondparameter indicating whether or not the cause for handover is solved,and transmit a reverse handover request message to the second basestation for the terminal to perform reverse handover to a target basestation, when the network node determines that a reverse handover isneeded, wherein the network node determines that the handover is needed,if the first base station is closed subscriber group (CSG) cell and thecause is an expiry of a CSG membership, and wherein the network nodedetermines that the reverse handover is needed, if the cause is anexpiry of a CSG membership and the second parameter indicates updatingof the CSG membership.